A dispositive treatise on jack-up rig units, rack-and-pinion type jacking systems, and rack chocks, including other forms of rack-teeth engaging devices, is available in U.S. Pat. No. 4,269,543 granted May 26, 1981 to J. L. Goldman et al.
The rigidification system of U.S. Pat. No. 4,269,543 includes a three-tooth rack chock element to "interdigitate and mate" with the teeth of the leg rack, yet teaches that the desired rigidification by "one simple tooth is possible." Most leg racks are fabricated by flame cutting which can, and usually does, develop racks that are out-of-tolerance. It is known that available leg racks, because of this imprecise cut, do not have identical tooth profiles and tooth spacing, i.e., pitch. This lack of precision does not affect known jacking systems since the pinion arrangement of such systems accomodates dimensional variations. Such a jacking system (rack-and-pinion type) is taught by U.S. Pat. No. 3,606,251 granted Sept. 20, 1971 to H. L. Willke et al. and reissued Feb. 14, 1978 as U.S. Pat. No. Re 29,539. However, because of these dimensional variations, it can not be assured that load bearing contact between the rack chock and the leg rack is made with more than one tooth since the rack chock cannot be custom fitted to the leg rack when the relative position of the moveable platform can and does vary over the entire length of the leg rack.
In a best mode of the invention, the chock system has a plurality of teeth that are moveable individually generally parallel with a leg rack of a jack-up rig unit for pre-locking alignment with the rack teeth pitch, and as a group into and out of locking engagement with the leg rack so that accurate and positive engagement can be made by each of the teeth with the mating root between adjacent ones of a plurality of leg rack teeth.